Identifying therapeutic targets for autosomal dominant polycystic kidney disease (ADPKD) will depend on a detailed understanding of disease pathogenesis. Mutations in PKD1 or PKD2 trigger incompletely understood signals that cause epithelial dedifferentiation, proliferation and cyst formation. Cysts lead to functional renal impairment through both nephron loss and peritubular fibrosis. Our laboratory originally identified Kidney Injury Molecule-1 (Kim-1) a type 1 membrane protein that is undetectable in normal kidneys, but undergoes strong induction after renal injury. Cells that express Kim-1 express mesenchymal markers such as vimentin and have increased proliferative rates. Strong expression of Kim-1 is seen in both human ADPKD kidneys and in the cystic kidneys of the PKD2(WS25/1) mouse, where its expression is associated with decreased complexity and quantity of basolateral staining for Na-K-ATPase. The interstitial cells surrounding Kim-1-expressing tubules demonstrate both high proliferative activity and staining for myofibroblast smooth muscle a-actin. In vitro studies indicate that Kim-1 promotes epithelial to mesenchymal transition (EMT). Heterologous Kim-1 expression drives down-regulation of E-cadherin, upregulation of vimentin, activation of the small GTPase Rho and increased paracellular permeability. We hypothesize that in ADPKD Kim-1 contributes to both cyst formation and renal fibrosis by driving EMT. Kim-1 may be downstream of PCI (the PKD1 protein product) which normally acts to suppress Kim-1 expression. In this case mutant PCI may induce Kim-1 and drive epithelial dedifferentiation. In the first aim, we will use live cell imaging and biochemical techniques to determine the effect of Kim-1 expression on PCI function and the PCI protein complex. Specifically, we will test the effect of Kim- 1 on E-cadherin stability in the adherens junction of renal epithelial cells, cilia structure and cilia-dependent mechanosensation. In the second aim we will develop a transgenic mouse overexpressing Kim-1 in proximal tubules using Cre-Lox technology. We will determine whether Kim-1 overexpression accelerates cyst formation and renal fibrosis in the context of PCI haploinsufficiency. Together, these studies will provide important information regarding the mechanisms of cyst formation in ADPKD, and will validate Kim-1 as a therapeutic target in ADPKD.